C3, c4, c2 cycles

Sometimes in C₃ plants, RuBisCO binds to oxygen molecules and the reaction deviates from the regular metabolic pathway. The combination of RuBP and oxygen molecules leads to the formation of one molecule of phosphoglycerate and phosphoglycolate. This pathway is called photorespiration. During photorespiration, no sugar or ATP molecules are synthesized, but just CO₂ is released at the expense of ATP and the whole process is futile.

However, C₄ plants do not undergo photorespiration due to their special mechanism to increase the CO₂ level for enzyme binding. During the Hatch and Slack Pathway, the C₄ acid, oxaloacetic acid (OAA) breaks down to release CO₂. This ensures the high concentration of intercellular CO₂. Thus, in C₄ plants, RuBisCO is more active as a carboxylase enzyme rather than as oxygenase. This is why C₄ plants have better productivity.